The invention relates to a device and a corresponding method for the controlled stepwise reduction of the gas flow Q supplied to a burner nozzle of a gas-fired cooking or baking appliance via a gas supply pipe.
Conventional cooking or baking appliances, e.g. gas cookers, gas cooking ranges or gas baking ovens, have one or more burners, in which the gas is mixed with atmospheric oxygen and burnt. The gas is fed to the burner via a gas supply pipe, which is supplied with gas by a gas mains, a gas tank or a gas cylinder. With a town gas supply system the feed pressure is about 8 mbar; however, it is subject to fluctuations and may fall to 4 mbar. The feed pressure is about 50 mbar in the case of cooking and baking appliances operated with camping gas.
The burners have a nozzle, which forms the essential flow resistance limiting the discharged gas flow and thus determines the maximum heating power of the burner when the latter is connected to the gas supply pipe. By contrast, the flow resistance in the gas supply pipe can generally be disregarded. However, the maximum heating power of the burner must be reduced by the user to the heating power required at a given time in practice. Hence it must be possible to reduce the heating power with the aid of a suitable control element at any time, in a simple way and to a value as close as possible to the desired or required heating power.
According to the state of the art, conventional continuous control valves are used to reduce the heating power of the burner. The gas flow is throttled by partially closing the valve, and the required gas throughflow and the required heating power are thus adjusted. In most cases the valves are adjusted manually. The setting accuracy of the valves is relatively small. Furthermore, proportional valves of this type also exhibit hysteresis in the control response, so that the throughflow depends not only on the valve position or the indication on the associated adjusting knob, but also on the direction in which the valve is actuated to adjust the required throughflow (i.e. opened or closed) and the length of the preceding adjustment path.
For this reason the user is generally not guided by the scale assigned to the valve, but changes the position of the valve until the required heating power, which he can evaluate from the size of the flame or the cooking or baking of the food, is achieved. By including the user equalizing these scale deviations in the control of the heating power it can be assumed that the setting accuracy and reproducibility of the gas flow are extremely small, and the flame size and heating power may thus vary considerably with the same setting of the controller or scale.
In applications where automatic or motor-driven adjustment of the gas flow is required, it is known that stepping motors controlled by a control circuit can be used to adjust the valves. However, this solution is technically complicated and cost-intensive. The problem that the proportional valves available or used exhibit hysteresis behavior also occurs in this case, so that with control of a specific valve position by means of the stepping motor according to the control direction and control path length different gas flows result. Reproducibly assigned heating powers are thus not achieved in the respective settings even in these cases.
A measuring and testing device for single adjustment of a gas heater, in which two gas pressure controllers connected in series, a programmable control system with the operating characteristic curves required for the respective adjustment cases and four pressure measuring instruments are provided, is known from the document DE 4225789 A1. Furthermore, a number of parallel branch pipes, each of which consists of a series connection of a solenoid valve and a reference nozzle, is provided. Only one of the branch pipes is opened to adjust the gas throughput for adjustment of the gas heater; a specific gas throughput is achieved by optional connection of several nozzles in parallel only in exceptional cases. This already known device is technically very complicated, so that even though it is suitable for balancing an adjustable throttle or an adjustable gas appliance pressure controller of a gas heater as part of the production checking system, it is not suitable for the permanent adjustment of the heating power of a gas-fired cooking or baking appliance by the user.
A blower burner, in which the quantity and ratio of gas and combustion air are controlled by two continuously controllable control valves and a balance controller, is known from U.S. Pat. No. 4,585,161. A further controllable auxiliary valve is connected in parallel with the controllable control valve in the gas supply. The degrees of opening of the gas valve and auxiliary valve to achieve a constant gas throughput are controlled by a control device. Sensors for measurement of the gas throughflow rates are required for this purpose.
A controllable gas burner of a gas cooker, in which one and the same mixing pipe of the gas burner is supplied by several switchable nozzles adjacent to each other, is already known from the document CH-303445. The individual nozzles are switched on and off by a common valve, the intermediate stages being realized by throttling by cross-section reduction preceding the individual nozzles in each case.
A gas regulator with a sequence of openings with different diameters, just one of which is opened for throughflow of the gas for each required heating stage, is already known from document FR-911.892.